At present, lead and lead alloy are used as anode during the process of extracting metals such as zinc, copper, nickel, manganese and chromium by hydrometallurgy. Anodes of this type have limited applications because of the various disadvantages such as: high cell voltage (3.4V to 3.8V); low current efficiency (75% to 88%); high energy consumption in electrowinning process (3400 kWh to 4200 kWh for one ton of Zn); short service life of the anode (half year to one year); the lead dissolves in the electrolyte and the resulting lead is deposited on the cathode subsequently resulting in a decrease in the purity of the deposit obtained. In order to reduce the energy consumption from the electrowinning of zinc, copper, nickel, cobalt, manganese and chromium and prevent the contamination of a cathode deposit, research and development of insoluble anode used in electrowinning process are carried out all over the world. Taking into account the current research and usage at home and abroad, there are mainly six types as follows:
(1) Lead-silver alloy (0.5 wt % to 1.0 wt % of silver) anode: although the manufacturing technique is simple, it is expensive due to the high content of silver. Therein, Pb—Ag—Ca ternary alloy anode and Pb—Ag—Ca—Sr quaternary alloy anode have advantages such as high strength, high corrosion resistance, long service life and low cost. However, when it is used, it causes local area corrosion and hard crusts on the surface of anode slime, which is hard to be removed, resulting in the increase of cell voltage, as well as great loss of silver and calcium when the anode is recycled.
(2) Titanium-based dimensionally stable anode with plating surface (coated with precious metal or oxide thereof), which has advantages of: overall dimensions stable, no short circuit problem caused by deformation and bending, high quality cathode products, light weight of the plate which is convenient for transportation and replacement. One of the preferred anodes of the group for electrowinning has been found to be Ti/IrO2(70%)-Ta2O5(30%) anode since the anode is more stable to the oxygen evolved during electrowinning. However, this anode is rather expensive due to the high cost of iridium and experience with the anode has shown that the presence of manganese ions in the electrolyte adversely affects the coating by precipitation of manganese oxides on the anode. Such manganese oxides do not show any electrocatalytic properties and is electrically insulating and therefore the anode becomes progressively disactivated, easily causing the peeling off of plating layer and short service life. In addition, titanium-based coating layer anode cannot be used in electrolyte that contains P ion; sometimes, metals in coating layer dissolves and causes the damage of the plate.
(3) New type inert lead dioxide anode: titanium, graphite, plastic and ceramic are usually chosen as the substrate material for this type of electrode. The anode was produced by applying to a titanium substrate after surface roughening treatment an electro-conductive under coating consisting mainly of metal oxides, then covering the undercoating with an intermediate coating consisting of a stress-free α-PbO2 deposit, and finally covering the intermediate coating with a top coating consisting of β-PbO2. However, the PbO2 anode prepared by electroplating as an insoluble anode has the following problems during use: a. PbO2 deposition layer cannot combine tightly with the surface of the electrode or the deposition layer is non-uniform; b. the PbO2 deposition layer is polyporous, rough and with a strong internal stress; c. PbO2 deposition layer is easily peeled off or eroded, resulting in a short service life; the PbO2 anode with fluorine-containing resin and/or inactive particles has a high cell voltage when it is used in hydrometallurgy.
(4) The anode obtained through inter-fusion process of molten salt electroless plating transitional layer, or electroplating methods, using light-weight metal aluminum as a core and lead alloy as an outer layer, which also has problems difficult to solve: the fluidity of the lead alloy and holes that may exist at some parts of the large size anode plate; grain boundary cracks on the plating layer, which allows oxygen generated during electrolysis process to pass the cracks and oxidize the aluminum matrix, forming alumina film with poor conductivity and causing the deterioration of the anode properties.
(5) Lead anode after surface pre-treatment. Lead-based alloy itself is unstable in sulfuric acid. When the lead-based alloy anode is put into the sulfuric acid solution, reaction occurs on the surface of the new anode rapidly, and the surface of anode is gradually covered by a nonconductive PbSO4 layer. The nonconductive PbSO4 layer blocks further erosion of the inner lead alloy while leads to the increase of the cell voltage of the anode. The high oxygen evolution potential on the lead surface leads to the generation of PbO2. Although α-PbO2 or β-PbO2 can be obtained by conditioning the lead anode surface through conventional methods, the α-PbO2 has an orthorhombic structure with poor conductivity and low hardness; the β-PbO2 has high oxygen evolution overpotential, leading to a high cell voltage. In addition, it cannot solve the lead into the cathode product, reduce the cathode product quality. In order to overcome the above problems, lead dioxide is replaced by manganese dioxide as the last outer layer. As an insoluble anode, manganese dioxide has a low oxygen evolution overpotential and save energy. However, most of the electrolytic manganese dioxide is obtained in sulfuric acid system, and the thickness needs to between 10 μm and 100 μm; the manganese dioxide plating layer is prone to fall off when the thickness is over 100 μm. In addition, when it is used in electrolyte containing chloride ion, the generation of Cl2 cannot be inhibited efficiently.
(6) Fence-like anode plate for metal extraction by hydrometallurgy, which has advantages of: improving the fluidity of the electrolyte, enhancing the efficiency and the quality of electrolysis metal collection, avoiding the disadvantage of touching the anode plate when the cathode plate is lifted, decreasing the material cost by using cheap aluminum matrix as the substrate. However, it still has disadvantages of: high cell voltage, short circuit due to the crystallization on the surface of conductive beam, the distribution of power lines affected by the insulating sheath, low yield of cathode product and short service life.
With the continuous exploitation of zinc resources, high-quality zinc concentrate is decreasing while impurity enriched zinc concentrate which is hard to smelt is increasing gradually, causing the entry of various zinc-containing materials that are not assured in the past into the recycling process. The purchase and utilization of high-chloride zinc concentrate and zinc-oxide powder at home and aboard is also one of the main reasons that leads to the seriously slump of technical indicators in production. Experiments show that when the content of chlorine in electrolyte is over 1000 mg/L, anode is eroded by chloride ion, which is oxidized to chlorate, the plate becomes thin and perforated, and the service life is shortened. Therefore, it has an important practical value to find a new type of energy-efficient inert anode with low cost, corrosion resistance, high conductivity, deformation resistance and long service life for the industrial production of hydrometallurgy.